Adjust the oven rack to the middle position. Preheat oven to 375 degrees F. Whisk egg yolks in medium size mixing bowl and set aside. In a medium saucepan, combine cornstarch, water, sugar, and salt. Whisk to combine. Turn heat on medium and, stirring frequently, bring mixture to a boil and boil for 1 minute. Remove from heat and gradually add hot mixture to egg yolks until warm. Return warm egg yolk mixture back into the saucepan and cook for 1 more minute. Remove from heat and gently stir in butter, lemon juice until well combined. Pour mixture into pie shell and top with meringue. Make sure meringue completely covers filling and that it goes right up to the edge of the crust. Bake for 10 to 12 min. or until meringue is golden brown. Remove from oven and cool on a wire rack. Make sure pie is cooled completely before slicing. Meringue: Place egg whites and cream of tartar in the bowl of a stand mixer. Beat egg whites until soft peaks form and then gradually add sugar and continue beating until stiff peaks form, approx. 1 to 2 min. Place quickly on top of the lemon filling and directly into the oven to avoid sinking meringue.